OK, for starters.... do you believe the Apostle Paul was a born again Christian???
1 Corinthians 9:27
I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
Castaway (G0096 in Strong's) = unapproved, rejected, worthless, reprobate.
This is the same greek word used in Romans 1:28 when the Lord turned those over to a reprobate mind who refused retain God in their knowledge and walk with Him.
This means, Paul believed that if he did not control himself and went back to walking after the fleshm HE would be unapproved, rejected, worthless, reprobate!
Obviously the Apostle Paul did not believe in "once saved, always saved" aka "eternal security"
Yes, I believe Paul was a born again Christian.
When he spoke of being rejected after testing (translated here "reprobate"), you assign that to mean that he would have lost his rebirth, becoming unsaved. However that puts you in direct contradiction to both Paul and John, in these passages:
Colossians 3:1-4 KJV
1) If ye then be risen with Christ, seek those things which are above, where Christ sitteth on the right hand of God.
2) Set your affection on things above, not on things on the earth.
3) For ye are dead, and your life is hid with Christ in God.
4) When Christ, who is our life, shall appear, then shall ye also appear with him in glory.
1 John 3:1-3 KJV
1) Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
2) Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.
3) And every man that hath this hope in him purifieth himself, even as he is pure.
In both of these passages, a clear and plain statement is made that those who have been born again - as I believe Paul was - will remain to be with Jesus when He appears in glory, and will remain to be transformed to be like Him when He appears.
You shall appear with Him in glory. You shall be like Him when you see Him. If these simple statements are shown false then the Bible is simply not true. But what about Corinthians, being reprobate?
Again, this means being rejected after testing. The "testing" isn't specified, as you've posted that particular verse but not more context.
1 Corinthians 9:22-27 KJV
22) To the weak became I as weak, that I might gain the weak: I am made all things to all men, that I might by all means save some.
23) And this I do for the gospel's sake, that I might be partaker thereof with you.
24) Know ye not that they which run in a race run all, but one receiveth the prize? So run, that ye may obtain.
25) And every man that striveth for the mastery is temperate in all things. Now they do it to obtain a corruptible crown; but we an incorruptible.
26) I therefore so run, not as uncertainly; so fight I, not as one that beateth the air:
27) But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
"Castaway", that is, adokimos, the negation of dokimos, "accepted after assay", that is, tested and approved. Castaway, reprobate, "tested and rejected".
So the question is, what is rejected, and what results from this rejection? This word is used in contexts such as a reprobate mind, reprobate concerning the faith, reprobate works. What is the context here?
Paul is here speaking of his efforts to bring others to Christ, and then goes on to compare this to a contest, and how the athlete strives for their crown. He too is seeking a crown, a better crown, and is harsh on himself, not like he's just "air-boxing", not a tentative runner, but he's harsh on his body, making it his slave, so that after preaching to others, he is not himself rejected.
Is he speaking here of his salvation, his rebirth? Or is he speaking of the reward of his ministry, that is, the effectual salvation of those to whom he preached, his crown?
1 Thessalonians 2:19 KJV
For what is our hope, or joy, or crown of rejoicing? Are not even ye in the presence of our Lord Jesus Christ at his coming?
However we are to interpret this passage, we know that it won't contradict passages such as,
1 John 3:1-3 KJV
1) Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
2) Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.
3) And every man that hath
this hope in him purifieth himself, even as he is pure.
Much love!